1. Field of the Invention
The present invention relates generally to a viscous fluid coupling device. In particular, the present invention is concerned with a pump plate assembly of a viscous fluid fan clutch for a vehicle.
2. Statement of the Related Art
A thermostatically-controlled viscous fluid clutch assembly for driving and rotating a vehicle cooling fan is well-known. A multi-bladed fan is removably secured to a body of the clutch assembly. The fan and clutch assembly are installed between an accessory pulley of a vehicle engine (typically the water pump pulley) and a radiator. The clutch assembly drives the fan at high speeds close to input speed when cooling is required and permits the fan to rotate at low speeds when cooling is not required. Thermostatic control of the fan through the clutch assembly reduces the load on an engine and the noise caused by fan rotation, resulting in horsepower gain and improved fuel economy.
Generally, a clutch plate, housed within the clutch assembly, having lands and grooves is mated to the body having complementary lands and grooves. A pump plate separates a pair of internally-contained chambers, a receiving chamber and a collecting chamber, from a reservoir. Gates in the pump plate permit selective flow of a viscous fluid from the reservoir to the receiving chamber and into a shear zone between the lands and grooves of the body and clutch plate. Fluid shear in the lands and grooves transfers input torque from the clutch plate to drive the body and the attached fan.
When cooling is not required, gates in the pump plate are closed and the fluid in the shear zone is pumped into the receiving chamber. Orifices in the pump plate permit passage of the fluid from the receiving chamber into the reservoir. The removal of a majority of the fluid from the shear zone substantially reduces the shear between the clutch plate and the body, thereby substantially reducing the rotation of the fan.
When an engine is not running, fluid in the reservoir may settle at an equilibrium level in a conventional clutch assembly. Fluid pressure may cause the migration of fluid from the reservoir into the receiving chamber through the pump plate orifices and into the shear zone. When an engine is next started, fluid that has migrated into the shear zone results in annoying high-speed operation of the fan. Such high-speed operation creates unwanted noise from the fan blades. Also, excessive rotation of the fan of a cold engine increases the engine warm-up period.
The art continues to seek improvements. It is desirable that a viscous fluid clutch assembly provide thermostatic operation of a fan when cooling is required. Furthermore, it is desirable that a clutch assembly prevent the migration of fluid from a reservoir to the shear zone when the engine is not in operation, thereby eliminating high-speed operation and unwanted fan noise when a cold engine is started.